Sains Ma1aysiana 25(3):115-130 (1996)                                                                                       Sains Bumi/

                                                                                                                                                                Earth Sciences

The Extension of the Bok Bak Fault in Northwest

Peninsular Malaysia: Location and Implications

Syed Sheikh Almashoor

Jabatan Geologi

Universiti Kebangsaan Malaysia

43600 UKM Bangi Selangor D.E.

ABSTRACT

The Bok Bak fault is a northwest-directed sinistral fault in northwest Peninsular Malaysia whose location was, by late 1980's, quite established. A postulated extension of 450 kilometres to the north-northwest has never been substantiated, but instead, speculated upon. The postulated extension in north Kedah, if there was any, had escaped detection of several mapping exercises, including modern approaches. The weak topography of the area and thick unconsolidated superficial deposits, apparently, are responsible for clouding the sensitivities of the modern methods. However, the conventional approach adopted in this study led to a successful discovery and definition of 82 kilometres of the Bok Bak fault in north Kedah and Perlis up to the Perlis (Malaysia)/Thailand boundary. It is in a dextral six-kilometre offset position with the established counterpart to its south. One significant fault­signature identified is a set of strike ridges which exhibit a left-lateral drag arrangement. Other fault-signatures include, fault planes and breccia, a sharp formational boundary, a spring locality, a lake, two linear rivers, a distorted river's course and a bent watershed line. One fault signature indicates that it is a wrench fault. With a zone of width 800 metres approximately and a total length of 215 kilometres, it is now established that the Bok Bak fault is a major fault in Peninsular Malaysia. Two pairs of correlatable rock units on opposite sides of the fault provide the first ever reliable displacement amount of about 10 km. It is discovered that the limestone hills (mogotes) running from north to central Perlis are not entirely of Chuping Formation. The last hill at the southernmost end (Bukit Ngulang) belongs to Setul Limestone Formation. The formational boundary between these hills, as implied by superficial fault-related features, is a fault contact; and is interpreted as a reliable signature of the Bok Bak fault. Based on similarities in hill-orientation, lithology and fossil contents, the Kodiang limestone mogotes in north Kedah are believed to be the continuation of the Chuping Limestone of central Perlis, but are now dispositioned by the Bok Bak fault.

ABSTRAK

Sesar Bok Bak ialah sesar ke kiri mengarah ke baratlaut yang terletak di baratlaut Semenanjung Malaysia. Pada akhir tahun lapan puluhan lokasinya sudah termaklum. Unjurannya yang sejauh 450 km ke arah utara-baratdaya hanyalah satu spekulasi dan tidak pernah dibuktikan. Unjurannya yang diandaikan di utara Kedah tidak dapat dikesan walau pun beberapa pemetaan telah dibuat, termasuk yang menggunakan pendekatan moden. Topografi kawasan yang lemah dan permukaan bumi yang diselimuti oleh bahan tak terkonsolidasi yang tebal dipercayai telah melemahkan kepekaan kaedah moden. Walau bagaimanapun pendekatan konvensional yang diguna dalam kajian ini berjaya mengesan sepanjang 82 km sesar Bok Bak di utara Kedah dan Perlis hingga ke sempadan Perlis/Thailand. Sesar yang baru dikesan ini terganjak ke kanan sejauh 6 km daripada bahagian sesar yang sudah termaklum di selatan. Satu tanda sesar yang penting yang dikenal pasti adalah satu set permatang jurus yang mempamerkan susunan seret ke kiri. Tanda-tanda sesar lain termasuk satah-satah sesar dan breksia, sempadan formasi yang tajam, satu lokaliti mata air, satu tasik, sungai-sungai lurus, sebahagian sungai yang tererot, dan garis legeh yang bengkok. Satu tanda sesar menunjukkan sesar ini adalah jenis mendatar. Dengan zon berkelebaran 800 meter dan kepanjangan sejauh 215 km, terbuktilah bahawa sesar Bok Bak adalah satu sesar major di Semenanjung Malaysia. Dua pasang unit batuan yang boleh dikorelasi dan yang terletak di sisi timur dan barat sesar, pertama kali membenarkan nilai alihan ke atas sesar dianggarkan, iaitu sebanyak 10 km. Kajian juga mendapati bahawa bukit-bukit batu kapur (mogot) yang berbaris daripada utara ke tengah Perlis bukan kesemuanya batuan Formasi Chuping. Bukit yang terselatan sekali (Bukit Ngulang) ternyata komponen Formasi Setul. Fitur-fitur di permukaan membayangkan bahawa sempadan formasi antara bukit-bukit tersebut ialah sempadan sesar; dan sempadan ini dianggap sebagai tanda sesar Bok Bak yang menyakinkan. Berdasarkan kesamaan pada orientasi bukit-bukit, litologi dan kandungan fosil, mogot-mogot Batu Kapur Kodiang di utara Kedah dipercayai adalah sambungan kepada Batu Kapur Chuping di tengah Perlis, tetapi sekarang telah teralih oleh sesar Bok Bak.

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